A more or less long service life of the electric circuits, depending on the particular application, is required for electronic products based on organic field-effect transistors (OFETs) and circuits to be commercially viable.
The best circuit properties are achieved using bottom-contact architecture (see H. Klauk, M. Halik, U. Zschieschang, F. Eder, G. Schmid, and C. Dehm, “Pentacene organic transistors and ring oscillators on glass and on flexible polymeric substrates,” Appl. Phys. Lett., vol. 82, p. 4175 (2003), which is incorporated herein by reference).
It has been found that transistor properties, for example of OFETs produced using bottom-contact architecture, are sensitive to moisture when various organic semiconductor compounds (e.g., pentacene, oligothiophenes, polythiophene derivatives) are used. This is based on the fact that in bottom-contact OFETs, the organic semiconductor layer is the last layer deposited (i.e., the top layer) and is therefore in direct contact with the environment, i.e., with atmospheric humidity.
The result of this is that water molecules, aided by the morphology of many organic semiconductor layers (for example pentacene as organic semiconductor does not form homogenous, amorphous layers, but rather forms microcrystallites, at the grain boundaries of which the layer thickness is only a few molecular layers) can get very close to or even penetrate into the charge carrier channel, where they can act as charge carrier traps and/or increase the free surface energy of the semiconductor/dielectric interface by their polar character.
Effects observed include, inter alia, a shift in the threshold voltage, a deterioration in the subthreshold swing, a reduction in the on/off ratio and an increase in the hysteresis of the transistors. These effects make it difficult to design integrated circuits based on organic transistors or make it altogether impossible to produce circuits that are able to function correctly.
The possible options for providing organic semiconductor layers with a protective layer that protects the semiconductor layers from environmental influences, in particular, moisture, are limited. The reason for this is the sensitivity of the organic semiconductor layers to organic solvents, from which suitable polymeric protective layers could be deposited, and the sensitivity of the organic semiconductor layers to thermal stresses, as occur during deposition of inorganic protective layers from the vapor phase, e.g., when using silicon oxide, silicon nitride, aluminum oxide.
There is a known variant for applying a polymeric protective layer to an organic semiconductor that has been developed in order to pattern organic semiconductor layers, i.e., to isolate individual transistors in integrated circuits from one another in order to avoid leakage currents between the transistors (see C. D. Sheraw, L. Zhou, J. R. Huang, D. J. Gundlach, T. N. Jackson, M. G. Kane, I. G. Hill, M. S. Hammond, J. Camps, B. K. Greening, J. Franc, and J. West, “Organic thin-film transistor-driven polymer-dispersed liquid crystal displays on flexible polymeric substrates,” Appl. Phys. Lett. vol. 80, p. 1088 (2002), which is incorporated herein by reference). This photographically patternable polymer formulation is based on the polyvinyl alcohol/ammonium dichromate (PVA/ADC) system and is applied from a neutral aqueous solution. Most organic semiconductors tolerate this aqueous system on account of their strongly hydrophobic character, i.e., the organic transistors remain able to function after the treatment, which contrasts with the treatment using organic solvents (see D. J. Gundlach, T. N. Jackson, D. G. Schlorr, and S. F. Nelson, “Solvent-induced phase transition in thermally evaporated pentacene films,” Appl. Phys. Lett., p. 3302 (1999), which is incorporated herein by reference).
After the treatment with this system, the same disadvantageous effects are observed (shift in the threshold voltage, deterioration in the subthreshold swing, reduction in the on/off ratio and increase in the hysteresis of the transistors) as occur under the action of moisture on a comparable, untreated substrate over the course of time.